Intravascular Ultrasound (IVUS) has become an important interventional diagnostic procedure for imaging atherosclerosis and other vessel diseases and defects. In the procedure, an IVUS catheter is threaded over a guidewire into a blood vessel of interest, and images are acquired of the atherosclerotic plaque and surrounding area using ultrasonic echoes. This information is much more descriptive than the traditional standard of angiography, which shows only a two-dimensional shadow of the vessel lumen. Some of the key applications of IVUS include: determining a correct diameter and length of a stent to choose for dilating an arterial stenosis, verifying that a post-stenting diameter and luminal cross-section area are adequate, verifying that a stent is well apposed against a vessel wall to minimize thrombosis and optimize drug delivery (in the case of a drug eluting stent) and identifying an exact location of side-branch vessels. In addition, new techniques such as virtual histology (RF signal-based tissue characterization) show promise of aiding identification of vulnerable plaque (i.e., plaque which is prone to rupture and lead to onset of a heart attack).
There are two types of IVUS catheters commonly in use: mechanical/rotational IVUS catheters and solid state catheters. In a rotational IVUS catheter, a single transducer consisting of a piezoelectric crystal is rotated at approximately 1800 revolutions per minute while the element is intermittently excited with an electrical pulse. This excitation causes the element to vibrate at a frequency dependent upon the particulars of the transducer design. Depending on the dimensions and characteristics of the transducer, this operating frequency is typically in the range of 8 to 50 MHz. In general terms, a higher frequency of operation provides better resolution and a smaller catheter, but at the expense of reduced depth of penetration and increased echoes from the blood (making the image more difficult to interpret). A lower frequency of operation is more suitable for IVUS imaging in larger vessels or within the chambers of the heart.
The rotational IVUS catheter has a drive shaft disposed within the catheter body. The transducer is attached to the distal end of the drive shaft. The typical single element piezoelectric transducer requires only two electrical leads, with this pair of leads serving two separate purposes: (1) delivering the intermittent electrical transmit pulses to the transducer, and (2) delivering the received electrical echo signals from the transducer to the receiver amplifier (during the intervals between transmit pulses). The IVUS catheter is removably coupled to an interface module, which controls the rotation of the drive shaft within the catheter body and contains the transmitter and receiver circuitry for the transducer. Because the transducer is on a rotating drive shaft and the transmitter and receiver circuitry is stationary, a device must be utilized to carry the transmit pulse and received echo across a rotating interface. This can be accomplished via a rotary transformer, which comprises two halves, separated by a narrow air gap that permits electrical coupling between the primary and secondary windings of the transformer while allowing relative motion (rotation) between the two halves. The spinning element (transducer, electrical leads, and driveshaft) is attached to the spinning portion of the rotary transformer, while the stationary transmitter and receiver circuitry contained in the interface module are attached to the stationary portion of the rotary transformer.
The other type of IVUS catheter is a solid state (or phased array) catheter. This catheter has no rotating parts, but instead includes an array of transducer elements (for example 64 elements), arrayed in a cylinder around the circumference of the catheter body. The individual elements are fired in a specific sequence under the control of several small integrated circuits mounted in the tip of the catheter, adjacent to the transducer array. The sequence of transmit pulses interspersed with receipt of the echo signals provides the ultrasound data required to reconstruct a complete cross-sectional image of the vessel, similar in nature to that provided by a rotational IVUS device.
Currently, most IVUS systems rely on conventional piezoelectric transducers, built from piezoelectric ceramic (commonly referred to as the crystal) and covered by one or more matching layers (typically thin layers of epoxy composites or polymers). Two advanced transducer technologies that have shown promise for replacing conventional piezoelectric devices are the PMUT (Piezoelectric Micromachined Ultrasonic Transducer) and CMUT (Capacitive Micromachined Ultrasonic Transducer). PMUT and CMUT transducers may provide improved image quality over that provided by the conventional piezoelectric transducer, but these technologies have not been adopted for rotational IVUS applications due to the larger number of electrical leads they require, among other factors.
There are many potential advantages of these advanced transducer technologies, some of which are enumerated here. Both PMUT and CMUT technologies promise reduced manufacturing costs by virtue of the fact that these transducers are built using wafer fabrication techniques to mass produce thousands of devices on a single silicon wafer. This is an important factor for a disposable medical device such as an IVUS catheter. These advanced transducer technologies provide broad bandwidth (>100%) in many cases compared to the 30-50% bandwidth available from the typical piezoelectric transducer. This broader bandwidth translates into improved depth resolution in the IVUS image, and it may also facilitate multi-frequency operation or harmonic imaging, either of which can help to improve image quality and/or enable improved algorithms for tissue characterization, blood speckle reduction, and border detection. Advanced transducer technologies also offer the potential for improved beam characteristics, either by providing a focused transducer aperture (instead of the planar, unfocused aperture commonly used), or by implementing dynamically variable focus with an array of transducer elements (in place of the traditional single transducer element).